On Tuesday, 8 May 2018 10:42:07 UTC+1, Graeme wrote:
Well, I think the battery itself is OK. Went for a run Thursday
afternoon to charge the battery, then left it until this morning
(Tuesday) with the earth lead disconnected. Reconnected and an easy
start, from what felt like a still fully charged battery.

To recap, if parked with the battery connected, the battery is almost
flat after 48 hours, as in not enough to turn the engine to start. The
courtesy light switch(es) were suspect, but same thing happens with them
disconnected.

I tried Harry's bulb trick again (small MES panel light bulb between
-ve battery terminal and earth strap), and nothing. Tried using an
ammeter. Nothing. Yet current must be flowing somewhere.
--
Graeme

A friend spent quite a few months replacing the main battery on his boat as he couldn't find a fully charged battery that would turn the engine over.
In the end he found out that the cable between the battery and the engine there was a 0.9V drop so he was only gettign 11V at the engine whereas previously he only ever measured the voltage from the battery which was 12V.
Once he replaced the cable the engine started, but the lenght of this cable was almost theb lenght of the boat, so unlikely to be the cause in a car.